Quarry blasting for rock, such as limestone, granite, and other igneous rocks conventionally uses ANFO as the explosive. ANFO is a mixture of approximately 94% ammonium nitrate and 6% fuel oil.
In quarry blasting, a plurality of boreholes are drilled in a predetermined pattern or array. For example, the holes are drilled in a 10 foot by 10 foot pattern, with 3-9 inch diameters and depths of 20-90 feet. A cast booster with a blasting cap is placed in the bottom of the hole, and ANFO is added into the hole up to level approximately eight feet from the surface. Small rock chips from 1/4-1/2 inch in size, commonly called stemming, is placed in the top of the hole to confine the ANFO. The boreholes are detonated sequentially so as to provide free faces toward which the broken rock moves.
The energy and power factors vary, depending upon the geological structures being blasted. For example, limestone requires a power factor of 2-5 pounds per ton.
ANFO is also used in open pit mining, for such minerals as coal, taconite, copper and gold. In open pit mines, the boreholes are typically 10-15 inches in diameter, drilled in a 28.times.28 feet pattern to produce 40-60 feet faces.
ANFO is a popular explosive in both quarry mining and open pit mining due to its low cost. However, ANFO has several limitations. When the boreholes are filled with solid columns of ANFO, only 60-70% efficiency is achieved as the detonation rises in the borehole. Accordingly, in such a straight ANFO shot, the 30-40% waste must be considered to avoid oversize material which is detrimental to the digging and crushing equipment used after the blast to process the shot rock. Also, such waste increases the cost of producing the shot rock.
Methods for overcoming the inefficiencies of solid ANFO shot and to enhance its action in the borehole have been developed. One such method is the use of solid AP propellant which has typically been used as a rocket fuel. Because of various nuclear disarmament treaties and the requirements that missiles be disarmed, this material has essentially become an excess material. It must be disposed of and traditionally has been disposed of as a waste by open air firing of the propellant motors or open burning of the propellant. However, these disposal methods are no longer viable because of environmental considerations.
U.S. Pat. No. 5,261,327 proposes the use of such solid AP propellant with ANFO as a blasting composition for quarry blasting. As disclosed therein, the solid AP propellant is a mixture of about 70% ammonium perchlorate, 20% aluminum and 10% binder.
However, the use of such solid propellant has become problematic because, firstly, the amount of the solid propellant remaining has diminished significantly. In addition, it is relatively expensive. Thus, even if one were to formulate additional solid propellant, its cost lends against its desirability for use in quarry blasting.